\documentclass[10pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.15}
\usepackage{mathrsfs}
\usetikzlibrary{arrows}
\pagestyle{empty}

\begin{document}

% Define colors
\definecolor{eqxurv}{rgb}{0.88,0.45,0.08} % orange
\definecolor{dtsfsf}{rgb}{0.83,0.18,0.18} % red
\definecolor{rvwvcq}{rgb}{0.08,0.40,0.75} % teal
\definecolor{qqqqff}{rgb}{0.00,0.00,1.00} % blue

\begin{tikzpicture}[line cap=round, line join=round, >=triangle 45,scale=1.5]

\begin{axis}[
  x=1.0cm,y=1.0cm,
  axis lines=middle,
  grid=major,
  xmin=-1, xmax=9.3,
  ymin=-0.8, ymax=9.6,
  xtick={0,...,9},
  ytick={0,...,9}
]

% Lines
\draw[very thick] plot[domain=-3:12] (\x,{\x});            % y = x
\draw[very thick, color=qqqqff] plot[domain=-3:12] (\x,{\x+1}); % y = x+1

% Piecewise red polyline
\draw[very thick, color=dtsfsf]
  (0,0) -- (3,0) -- (3,1) -- (4,1) -- (4,5) -- (8,5) -- (8,8);

% Orange extension
\draw[very thick, color=eqxurv]
  (4,5) -- (4,9) -- (7,9);

% Labels for lines
\node at (3,2.3) {$y = x$};
\node[color=qqqqff] at (2.2,4.2) {$y = x+1$};

% Points
\fill[rvwvcq] (0,0) circle (2.5pt) node[below right] {$(0,0)$};
\fill[rvwvcq] (8,8) circle (2.5pt) node[below right] {$(n,n)$};
\fill[rvwvcq] (7,9) circle (2.5pt) node[above] {$(n-1,n+1)$};

\end{axis}
\end{tikzpicture}

\end{document}
